The start-up of a highly selective silver-based catalyst for ethylene oxidation requires a special procedure. The catalyst, especially when it comprises Re as a promoter, requires an initiation period before it is able to give the expected higher performance. In the prior art, Lauritzen, U.S. Pat. No. 4,874,879 and P. Shankar, U.S. Pat. No. 5,155,242, disclosed the prechloriding of fresh Re-containing catalyst before adding oxygen to the feed. In these disclosures, the catalyst was, initially, pre-chlorided with a feed containing ethylene, methane, and ethyl chloride. This was followed by adding oxygen to the feed and the reaction temperature was kept below 273° C. Eventually several adjustments to the reaction conditions were made in order to get the optimum performance. This pre-chloriding step, was claimed to enhance the activity of the rhenium-containing catalysts, and allow start-up at low temperatures.
A further start-up process is disclosed in J. Lockemeyer, U.S. Patent Application Publication No. 2004/0049061. Particularly, the J. Lockemeyer publication claimed a method for improving the selectivity of a supported highly selective epoxidation catalyst, comprising silver in a quantity of at most 0.17 g per m2 surface area of the support, via contacting the catalyst, with a feed comprising oxygen at a catalyst temperature above 250° C. for a duration of up to 150 hours. There was no mention in this disclosure of a moderator soaking step.
Evans, U.S. Pat. No. 7,102,022, claimed a method for the start-up of a process for the epoxidation of an olefin, comprising a silver-based highly selective epoxidation catalyst, via contacting the catalyst bed with feed comprising oxygen. In this treatment, the temperature of the catalyst bed was above 260° C. for a period of at most 150 hours. In addition to oxygen, one or more components selected from the olefin, carbon dioxide, the inert gases and reaction modifiers, such as organic halides can be present in the feed. However, it was stated that the presence of these additional components in the feed is not considered to be essential to the invention. However, when the feed comprised the addition of an organic halide, its concentration was in the range of from 1 to 30 ppm calculated on the basis of the halogen content, relative to the total feed. There was no mention in this disclosure of a moderator soaking step.
The aforementioned publications disclose start-up procedures that may be effective under some circumstances for certain catalysts. However, because of the importance for operating highly selective catalysts under optimum performance conditions, there is a continued need to develop new and improved methods for the start-up of these catalysts.